Kinetic precipitation of solution-phase polyoxomolybdate followed by transmission electron microscopy: A window to solution-phase nanostructure

This study aimed to elucidate the structural nature of the polydisperse, nanoscopic components in the solution and the solid states of partially reduced polyoxomolybdate derived from the {Mo₁₃₂} keplerate, {(Mo)Mo ₅}₁₂-{Mo₂ acetate}₃₀. Designer tripodal hexamine-tris-crown ethers and nanoscopic molybdate coprecipitated from aqueous solution. These microcrystalline solids distributed particle radii between 2-30 nm as assayed by transmission electron microscopy (TEM). The solid materials and their particle size distributions were snap shots of the solution phase. The mother liquor of the preparation of the {Mo₁₃₂} keplerate after three days revealed large species (r=20-30 nm) in the coprecipitate, whereas {Mo₁₃₂} keplerate redissolved in water revealed small species (3-7 nm) in the coprecipitate. Nanoparticles of coprecipitate were more stable than solids derived solely from partially reduced molybdate. The TEM features of all material analyzed lacked facets on the nanometer length scale; however, the structures diffracted electrons and appeared to be defect-free as evidenced by Moiré patterns in the TEM images. Moiré patterns and size-invariant optical densities of the features in the micrographs suggested that the molybdate nanoparticles were vesicular.